1. Field of the Invention
The present invention relates to an apparatus and method for performing simple fluorescence spectrometry using a UV LED light source in a downhole environment to estimate sample clean up and API gravity based on the spectral response over time. Tracers are added to enhance detection of oil and water based mud.
2. Background of the Related Art
Fluorescence analysis has been performed on cuttings or cores obtained during the drilling of wells to determine the presence of hydrocarbons in pore fluid. An example of such a technique can be found in U.S. Pat. No. 4,690,821. In techniques such as these, cuttings or cores are cleaned to remove any drilling fluid products, which might otherwise interfere with the analysis. The samples are crushed and extracted with a solvent, which is then analyzed. Alternatively the sample is irradiated directly and the fluorescence analyzed. While this technique can provide reasonably accurate analysis of the pore fluids, there are certain drawbacks. Cores are relatively expensive to obtain and must be returned to the surface for analysis. Also, since cores are only taken from specific locations, it is possible that hydrocarbon-bearing formation can be overlooked. Cuttings are obtained continuously in drilling, but have the disadvantage that it is not possible to determine at the surface exactly where the cuttings originate downhole, making the identification of hydrocarbon-bearing formations difficult. Also, cuttings give no accurate indication of the extent of any hydrocarbon bearing formations. Recent innovations have concentrated on performing fluorescence experiments in a downhole environment.
U.S. Pat. No. 5,912,459 by Mullins et al. titled Method And Apparatus For Fluorescence Logging discloses a method comprising illuminating a borehole with light from a source within a tool and detecting any fluorescent radiation with a detector in the tool and analyzing the fluorescent radiation to determine the presence of hydrocarbon in the formation. Preferably, the borehole wall is illuminated and fluorescence detected through a window in the tool which is pressed against the borehole wall. The window is typically pressed against the borehole wall with sufficient force to displace any mudcake for a substantial time, as the tool is moved through the borehole. Pressing the window against the borehole wall minimizes rugosity effects, assuming low rugosity.
PCT application (International Publication Number WO 01/20322 A1) discloses a method of fluorescence spectrometry for predicting the asphaltene precipitation onset pressure in a downhole formation. The invention of this patent comprises illuminating and measuring an isolated sample at several pressures. As asphaltenes precipitate, they induce significant optical scattering. Asphaltene precipitation is detected as a sharp reduction of transmitted light and a large increase in the light scattering strength of the sample. WO 01/20322 teaches fluorescence as a determination of contaminants only. Thus, there is a need for a method and apparatus for determining oil properties and to further oil sample purity using fluorescence.
A downhole environment is a difficult one in which to operate a sensor. Measuring instruments in a downhole environment need to work under conditions of limited space within a tool's pressure housing, at elevated temperatures, and they need to withstand shock and vibration. Thus, there is a need for a simple but robust fluorescence spectrometer suitable for operation in a downhole environment.